1. Field of the Invention
The present invention relates to a method for preparing .alpha.-Si.sub.3 N.sub.4, that is, the alpha form of silicon nitride, and more particularly to .alpha.-Si.sub.3 N.sub.4 powder of high quality which is obtained in a high yield and a consistent yield.
2. Description of the Prior Art
It is known that sintered silicon nitride-yttrium oxide or magnesium oxide (Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 or Si.sub.3 N.sub.4 -MgO) materials possess excellent mechanical strength and heat resistance, and therefore have been used in high temperature gas turbine engines. However, when the conventional Si.sub.3 N.sub.4 sintered products are used in practice as materials which are subjected to high temperatures and high stresses, their physical and chemical stabilities and reliability at high temperatures are absolutely essential requirements. Their thermal and mechanical properties, which are particularly important factors, are greatly affected by the nature of the starting materials and the quantities of impurities which these materials contain. Moreover, with regard to the silicon nitride it is desirable that it should contain as much .alpha.-Si.sub.3 N.sub.4 powder as possible. Especially desired is a finely divided .alpha.-Si.sub.3 N.sub.4 powder for use in sintering materials.
In the past, Si.sub.3 N.sub.4 powder has been synthesized by the following methods.
(1) 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 PA1 (2) A vapor phase reaction in which silicon tetrachloride or silane is reacted with ammonia as starting materials 3SiCl.sub.4 +4NH.sub.3 .fwdarw.Si.sub.3 N.sub.4 +12 HCl, and the like; PA1 (3) A method of nitridizing SiO.sub.2 obtained by reducing silica (SiO.sub.2) with carbon in the following stoichiometric ratio 3SiO.sub.2 +6C+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6CO, and the like.
In the case of method (1), the nitridization of Si is an exothermic reaction, and therefore the process must be carefully conducted so as to carefully control the generation of heat. For example, the Si which is commercially selected for the reaction is comparatively coarse-grained powder, and therefore, fine grinding is generally conducted after nitridization. Therefore, the admixture of impurities into the product during the grinding process is unavoidable, and although there is no objection to the use of this material for refractory materials in general, such as firebricks, it is not suitable for high temperature gas turbines.
The process of reaction (2) yields a product which is suitable, for instance, for the surface coating of semiconductor elements and the like, but it cannot be regarded as suitable for the mass production of inorganic refractory materials.
In the case of reaction (3) thoroughly purified SiO.sub.2 powder and carbon (C) powder must be used as starting materials, and there is also the disadvantage that the product produced by reacting stoichiometric quantities of SiO.sub.2 and C comprises a mixed system of .alpha.-Si.sub.3 N.sub.4, .beta.-Si.sub.3 N.sub.4 (beta form of silicon nitride), silicon oxynitride (Si.sub.2 ON.sub.2), silicon carbide (SiC) and the like. Moreover, the yield of .alpha.-Si.sub.3 N.sub.4 is low. In other words, this reaction system has the advantage that the reaction procedure is relatively easy, but the yield of .alpha.-Si.sub.3 N.sub.4 product is low, and therefore the method is not preferred in practice.
A need, therefore, continues to exist for a method by which high quality .alpha.-silicon nitride can be manufactured in high and consistent yield.